6 NATURAL HISTORY OF BIRDS. 



we liamincr caivfiilly the (leup lilue feathers of a macaw, the liliic color immediately Jis- 

 ai)j)eai"8, and the injured part looks gray or brownish, according to the underlying pig- 

 ment. Some green parrot feathers, when treated in a similar way, become yellow, 

 since this is the color of their ]iigment. Here we have the explanation of the ilark 

 appearance of the abraded jtarts of feathers of jiarrots and other brightly colored 

 birds. 



The gloss of feathers, indejicndent «l the color itself, is the result of their sm-face 

 being smooth and jiolished, while the metallic lustre is due to a transparent sheath 

 which acts like a jirism, a fact ascertained by Mr. Gadow. The theory of the metallic 

 lustre being due to structure of a prismatic nature originated, however, with Professor 

 IJ. Ahum. 



We mentioned above that the seasonal shedding of feathers or of their edges 

 usually causes a change in the color of the jilumage. In some birds we distinguish 

 summer and winter plumages, in otliei-s nuptial and ])ost-nuptial garbs, and in some 

 ptarmigan may be observed even four more or less distinct attires nearly corre.sjtonding 

 to the four seasons. 



There are also some interesting relations connected with the similarity and dissimi- 

 larity ill color between the two sexes, and between the adults and the young. Tlinii^di 

 it might seem to be the original arrangement, or perhaps just, therefore, young birds 

 and the adults of both sexes and at .'ill seasons are comparatively seldom tpiite alike. 

 The Procellarida', or i)etiels, may be ipioted as an example, besides several others. If 

 the adults of both se.xcs, for some reason or another, have developed alike se.ason.al 

 colors, the first ]ilumage of the young is very often like that which the parents assume 

 about the same time, — that is to say, their post-nuptial or winter dress. In such a 

 case the young birds un<leigo a change in the spring similar to that of the old ones ; 

 many of the auks (Alcidie) demonstrate this rule. Whenever one of the adults, no 

 matter what sex, is more richly colored than its mate, the young usually resemble the 

 more ])laiiily colored of the jiarents; this rule is followed by a great many, perhaps 

 the majority of birds, but exeei)tioiis and many modifications occur. We are, bow- 

 ever, justified in making this generalization, tiiat species in v. liich both parents differ 

 materially from the iilumage of the young are still more sjieeialized as to color than 

 the foregoing categories; for we may without hesitation take for granted that the 

 plumage of the young is the more generalized, and that the amount of specialization 

 is in ]iroportion to the dejiarlure from the first garb. It follows that weh.ave to go to 

 the birds in the later plumage, or in that more like it, whenever we wish to ascertain 

 the relationship of different forms. It will, therefore, be necessary to arrange the 

 species according to the characters furnished by the young, or jdain-colored females, 

 and not by the secomlary, often highly specialized, structure of the males, if we aim 

 at a natural classification based upon affinities. It will seem as if there may be a 

 possibility of finding out the relation between the different classes of j)luniages, so 

 thatit might be deduced whether one kind of plumage in a given case — for instance, 

 a barred ors]iotted one — is a more specialized condition than another, say a striped or 

 plain dress; but no investigations, covering a suHicieiit number of species of all orders 

 and from all parts of the world, have been made as yet, without which all generaliza- 

 tions and speculMlions are jiremature and next to valueless. 



Finally, Me have to consi.ler a color jiroblem which has only come forward of late, 

 and which still aw.iits its solution. There has been invented a name for the jihenoin- 

 enon, and we are accnstomed to call it dichromatism, but of its true nature and its 



